1. Field of the Invention
The present invention relates to a millimeter wave frequency band optical oscillator used for an oscillating frequency signal source of millimeter waves transmitted from a repeater to a wireless subscriber in a millimeter wave wireless subscriber communication system for a next-generation high speed wireless internet service and, more particularly, to a modulation frequency tunable optical oscillator capable of generating a dual laser mode by using a loop mirror and a pair of optical fiber grating mirrors.
2. Discussion of Related Art
Researches and developments of a communication technology using millimeter wave bands have been made in international and domestic communication technology institutes in order to provide subscribers with a service of high capacity data of about 100 Mbps by using a next-generation (i.e., 5th generation or less) personal wireless communication technology. In the millimeter wave band, the wireless communication should be serviced within a relatively short distance of about 200 m or less due to their high atmosphere attenuation. Therefore, subscribers should be located in the vicinity of a repeater, and extremely high frequency optical signals of a millimeter wave band are used to send information to the repeater via optical waveguides. In this case, an extremely high frequency optical oscillator is used for a signal source of oscillating frequencies for generating extremely high frequency optical signals.
Since five or six years ago, researches and developments of international or domestic technologies of this field have been focused on two modulation types, a semiconductor optical modulation for an extremely high frequency and a resonator-self modulation. In the semiconductor optical modulation type, a frequency band of 40 GHz or less is used, whereas in the resonator-self modulation type, a frequency band of 60 to 80 GHz is used to develop an optical fiber oscillator having a complex resonator structure.
According to Korean Patent Application No. 2002-3529, filed in Jan. 22, 2002, a ring resonator having a fiber optical grating mirror was proposed. Also, a laser light source capable of generating an extremely high frequency based on the ring resonator structure was developed by using the similar concept.
FIG. 1 is a schematic diagram showing a modulation frequency tunable optical oscillator using an optical fiber ring resonator, which comprises an amplifying optical fiber of 3 m for amplifying light beams, a polarization controller, a linear polarizer for changing optical wavelengths, an optical fiber grating mirror, an optical orientation indicator for isolating optical paths, an optical fiber grating mirror, a 10% coupler for obtaining output light beams of 1,530 nm or less, and a distributed compensation optical fiber of 4 m for generating non-linear polarization effects.
However, according to the conventional optical oscillator described above, since a laser mode 2 in which a light beam travels round a pair of optical fiber grating mirrors has a double resonant length in comparison with a laser mode 1 in which a light beam passes through them, so that birefringence and high variation of the resonant frequency happen. As a result, the frequency for modulating two modes becomes high but the width needed to adjust the polarization controller (polarized condition) becomes relatively narrow in order to oscillate two modes at the same time.